Coal slag universal fluid

ABSTRACT

A composition suitable for drilling and cementing comprising coal slag, water and drill solids, the components of the drilling fluid being chosen so as to have a dual functionality in promoting the drilling fluid and thereafter in being functional constituents of a cementitious slurry. Also, a method for drilling using a drilling fluid containing coal slag so as to lay down a filter cake which is settable and which is compatible with a subsequent coal slag cementitious slurry.

BACKGROUND OF THE INVENTION

This invention relates to drilling fluid compositions.

The drilling of boreholes is generally carried out using a rotarydrilling process. The rotary drilling of a borehole is accomplished byrotating a drill string having a drill pipe and a drill bit at its lowerend. Weight is applied to the drill bit while rotating to create aborehole into the earth. The drill string is hollow and sections areadded to the drill string to increase its length as the borehole isdeepened. This rotary drilling process creates significant amounts offriction which produces heat along with fragments of the strata beingpenetrated. The fragments of the strata must be removed from theborehole and the drill bit must be cooled to extend its useful life.Both of these necessities are accomplished by the circulation of a fluiddown through the drill string and up to the surface between the drillstring and the wall of the borehole.

Once the borehole has been drilled to the desired depth, it may bedesirable to isolate the separate areas, zones or formations transversedby the borehole. For extraction of fluids from formations, a conduit(casing) must be inserted into the borehole extending from the surfacedownward, and liners may be hung inside the casing.

At this point it becomes necessary to fill the annulus between thecasing and the borehole wall or between the liner and casing with amaterial which will seal the annulus (interfacial sealing) to inhibitcommunication between various formations penetrated by the wellbore andwhich will provide structural support for the casing or liner. This iscommonly referred to as primary cementing.

Bonding of the cement to the casing and borehole surfaces is critical toproviding an effective seal in the annulus and for providing support forcasings. Under most conditions, the bonding of cement to casing isachieved through contact of the cement particles with the surface of thecasing. The resulting region of contact provides a mechanical interfacewhich impedes movement of the casing due to high frictional forces. Afluid seal between cement and casing is also effected by the closecontact of the cement particles at the casing surfaces which results ina region of very low effective permeability that prevents fluidmigration along the interface.

Bonding between the cement and borehole wall is also achieved throughcontact of the cement particles with the formation or drilling fluidfilter cake commonly deposited at the borehole wall during the drillingof the borehole. However, bonding or interfacial sealing between thecement and borehole surfaces is not readily achievable. Cowan and Hale,U.S. Pat. No. 5,020,598 (Jun. 4, 1991) broadly disclose improved cementto casing sealing through the addition of a polyalcohol.

Generally, the borehole into which the casing or liner is introduced isfilled with drilling mud. Conventional Portland cement and conventionaldrilling muds are generally incompatible. Thus, a mixture ofconventional Portland cement and conventional drilling mud may not setup into a strong cement. In addition, the viscosity of such mixturesbecomes uncontrollable and may either become too viscous to pump or mayget thinner.

At the completion of drilling, the used drilling fluid is displaced fromthe borehole using some means to keep it separate from the cement tofollow. This creates two problems. First, the means developed by theindustry to keep the drilling fluid separate is relatively complex,involving the use of a landing collar and a pair of wiper plugs. Inaddition, the thus-displaced drilling fluid must be disposed of. Wyantet al, U.S. Pat. No. 3,499,491 (Mar. 10, 1970) proposed a partialsolution to this problem by mixing a cementitious material such asPortland cement with powdered sodium silicate glass and a treateddrilling fluid. While this does solve the problem of drilling fluiddisposal since the drilling fluid is incorporated into the cement, itnecessitates the use of extraneous components in order to achieve asufficient degree of compatibility to make the cement work at all.

It would be desirable to have a drilling fluid where most or all of thecomponents have both a drilling fluid function and a cementitiousfunction. It would also be desirable to have a cementitious slurry madefrom a drilling fluid wherein all of the ingredients are compatible.Even where cements can be made by adding cementitious materials todrilling fluids, ingredients in the drilling fluid adversely affect thefinal cement even when they can be rendered sufficiently compatible tobe operable. Peterson U.S. Pat. No. 4,780,220 (Oct. 25, 1988) disclosesa conventional drilling fluids containing a polyglycerine component.Tragesser, U.S. Pat. No. 3,557,876 (Apr. 10, 1969), discloses variouspozzolans in drilling fluids and, in combination with materials such ascalcium oxide or calcium hydroxide (lime), in cementitious materials.Hale and Cowan, U.S. Pat. No. 5,058,679 (Oct. 22, 1991), disclose blastfurnace slag to solidify an aqueous drilling fluid. Phillip et al, U.S.Pat. No. 4,756,761 (Jul. 12, 1988), discloses modification of coal slagwith lime to produce a cementitious material which can thereafter beactivated with lime.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a universal drilling fluidwhich is transformable to a cement.

It is a further object of this invention to provide a drilling fluidhaving components characterized by dual functionality in promoting boththe drilling operation and the cementing operation.

It is yet a further object of this invention to provide a universalfluid transformable into a cement without the addition of extraneouscompatibility additives.

It is yet a further object of this invention to provide a universalfluid that allows easy control over the setting time.

It is yet a further object of this invention to improve interfacialsealing.

It is yet a further object to provide a drilling process to lay down asettable filter cake. In accordance with this invention, a borehole isdrilled using a drilling fluid comprising water, drill solids and coalslag.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that a drilling fluid transformable to a cement can beproduced utilizing components which are characterized by (a) dualfunctionality in promoting both the drilling operation and the cementingoperation and, (b) compatibility with each other. Such a universal fluidavoids the problems of incompatibility and the necessity for addingextraneous compatibilizing agents and further avoids a compromise on thequality of the drilling fluid and the cement.

Definitions

In this description, the term "universal fluid" means a coalslag-containing aqueous composition suitable for drilling which, onactivation, sets up into a cement.

By "cementitious slurry" is meant a slurry comprising coal slag andingredients which cause the slurry to harden.

By "direct fluid contact" between the displacement fluid and thecementitious slurry is meant that the displacement fluid directlycontacts the upper surface of the column of cementitious slurry asopposed to having a solid wiper plug and/or spacer fluid disposedbetween the cementitious slurry and the displacement fluid. By "directfluid contact" between the cementitious slurry and the drilling fluid ormud is meant that the cementitious slurry directly contacts the uppersurface of the column of drilling fluid or mud as opposed to having awiper plug with a rupturable diaphragm and/or spacer fluid disposedbetween the cementitious slurry and the drilling fluid or mud.

The term "pipe" means either a casing or a liner.

The term "primary cementing" refers to any cementing operation wherein acementitious slurry is passed into an annulus surrounding a pipe andthus encompasses both the cementing of casings wherein the annulus isbetween the casing and the borehole wall and the cementing of linerswhere the annulus includes an annulus between the liner and the casing.

By "activator system" is meant either a single activator or a mixture ofactivators.

By "soluble", as it relates to polyalcohols, is meant a materialcontaining alcohol groups and generally ether linkages whichsufficiently soluble in water at room temperature that at least 80 gramswill dissolve in 100 grams of water. Lower molecular weight and/or a lowratio of ether linkages to alcohol groups tend to make the polyalcoholsmore soluble.

By "insoluble", as it relates to alcohols, is meant a materialcontaining alcohol and generally ether linkages which, generally becauseof a high ratio of ether to alcohol linkages, or the presence ofhydrocarbon chains from dihydric monomers, is substantially insoluble inwater at room temperature. Higher molecular weight also tends to makethe alcohols more insoluble as does the use of the small amount of unitsfrom dihydric alcohols having a chain length of greater than 3 carbonatoms.

By "cuttings" stabilizer is meant a material which inhibitsdisintegration of cuttings which is indicative of a wellbore or shalestabilizer.

By "borehole stabilizer" or "wellbore stabilizer" is meant an additiveor system of additives that reduces the stress state of the wellboreand/or modifies the wellbore such that strength of the formation isenhanced or chemically passivates clays in formation pores to reduceformation damage (reduced effective permeability).

By "shale stabilizer" is meant an additive or additive system whichstabilizes cuttings which is indicative of the ability to stabilize aborehole (wellbore).

As used herein "down" or "in" as it relates to a drill string or casingmeans in a direction toward the farthest reach of the borehole eventhough in some instances the borehole can be disposed in a horizontalposition. Similarly, "up" or "out" means back toward the beginning ofthe borehole.

Universal Fluid Composition

The universal fluid comprises:

a) an aqueous medium,

b) coal slag,

c) drill solids,

d) generally a fluid loss additive,

e) generally a rheology control agent (viscosifier) and/or solidssuspending agent,

f) optionally, a retarder,

g) optionally, a weight material (as needed),

h) optionally, a shale stabilizer (as needed),

i) optionally, a deflocculant (as needed).

The above description does not, however, dictate that the compositionwill have at least nine ingredients just for the drilling function. Itmay have more or less. Another novel feature of this invention is theuse of components which (a) have a dual functionality in that they havean important drilling fluid function and an important cementitiousslurry function, and (b) may have two or more drilling fluid functionsand/or two or more cementitious slurry functions. For instance, asilicate, if present, functions as a viscosifier and a shale stabilizerin the drilling fluid and then continues to function as a shalestabilizer in the subsequent cementitious slurry as well as acting as anaccelerator as is discussed in more detail hereinbelow. A polyalcohol,if present, also functions as a retarder, a shale stabilizer and as afluid loss additive in the drilling fluid and then functions as arheology control agent in the subsequent cementitious slurry as isdiscussed in more detail hereinbelow. A polyalcohol, if present, alsofunctions in combination with salt to give particularly good shalestabilization as is discussed hereinafter in connection with the aqueousmedium. This is particularly true with the mixtures of soluble andinsoluble polyalcohols which are discussed hereinafter in connectionwith the aqueous medium.

Aqueous Medium

The term "aqueous medium" is intended to encompass both fresh water andsalt water, including any fluid having water as the continuous phase,including oil-in-water emulsions, as well as essentially oil-free waterbased drilling fluids.

It is generally desired that the drilling fluids use water containingdissolved salts, particularly sodium chloride. In these instances, 0.1to 26 wt %, preferably 3 to 20 wt % sodium chloride based on the weightof the continuous phase may be used. In some instances, 5 to 20 wt % maybe preferred. One suitable source is to use seawater or a brine solutionsimulating seawater. The strength of the resulting cement is actuallyenhanced by the salt which is contrary to what would be expected in viewof the intolerance of Portland cement to brine. Various salts, includingorganic salts, are suitable for use in the drilling fluid used in thisinvention in addition to, or instead of NaCl, including, but not limitedto, MgCl₂, NaBr, KCl, CaCl₂, NaNO₃, NaC₂ H₃ O₂, KC₂ H₃ O₂, NaCHO₂,CsCHO₂ and KCHO₂. Sodium chloride is usually preferred, as noted above.These salts can be used, if desired, from 0.1 wt % up to the saturationpoint under the conditions employed.

The use of a salt solution such as seawater is particularly advantageousbecause salts such as sodium chloride act as a shale stabilizer in thedrilling fluid in addition to enhancing the strength of the cement asnoted hereinabove.

The salt modification of the aqueous phase to lower the water activityand increase the ionic strength results in stabilized cuttings as wellas actual wellbore stabilization.

Coal Slag

Coal slag is an essential ingredient of the universal fluid compositionused in this invention. It serves as a latent cementitious component inthe drilling fluid and further serves in the cementitious slurry as thecementitious constituent.

However, in the absence of activators it tends not to begin settingduring drilling. Thus, the coal slag is a desired and essentialingredient in the drilling fluid for three reasons. First, it allowslaying down a settable filter cake during drilling. Second, it providesa compatible residual material in the borehole for the cementitiousslurry. Third, because of its tendency not to set during the drilling,but to set rapidly in the presence of an activator, it gives excellentcontrol over the entire drilling and cementing operation.

By "coal slag" is meant the hydraulic refuse from either thecarbonization of coal or the burning of coal (as opposed to the smoke orfly ash). The coal can be anything in the series from peat, brown coaland lignite, sub-bituminous coal, bituminous coal, to anthracite coal.

By the term "carbonization" is meant to encompass coke production, coalgas production, coal tar production and the production of lighterhydrocarbons. One preferred source of coal slag is coal gasificationprocesses such as are described in Alpert et al, U.S. Pat. No. 5,091,349(Feb. 25, 1992), the disclosure of which is hereby incorporated byreference. Slag from the Lugri process for coal gasification is aspecific example of applicable coal slag. A suitable coal slag isavailable from the Santrol Division of Fairmont Minerals under the tradename `BLACK MAGNUM`. This is a water-quenched slag from the burning oflignite coal for heat.

Preferably, the coal slag used in this invention has a particle sizesuch that it exhibits a Blaine specific surface area between 2,000 cm²/g and 15,000 cm² /g and more preferably, between 3,000 cm² /g and15,000 cm² /g, even more preferably, between 4,000 cm² /g and 9,000 cm²/g, most preferably between 4,000 cm² /g and 8,500 cm² /g.

If desired, from 1 to 50 wt % of the coal slag can be replaced withblast furnace slag and/or fly ash, i.e., the weight ratio of the blastfurnace slag and/or fly ash to coal slag could be from 1:99 to 50:50.

Retarder

A retarder is generally not required in the universal fluid compositionof this invention since the coal slag will not set under normal drillingconditions without the presence of an activator. Organic compounds ingeneral, and more specifically, low molecular weight organic acids, aresuitable retarders, however, if a retarder should be desired.Lignosulfonates, including both chrome lignosulfonate and chrome-freelignosulfonate, can serve as retarders.

If used, retarders can be used in an amount within the range of 0.1 to30, preferably 0.1 to 20 volume percent based on the volume of thedrilling fluid.

Retarders are generally compounds which have OH⁻, COOH, BO₃ or BO⁻ ₄functional groups which are a part of or can be released from thecompound in solution. Chelating agents are also retarding agents. Suchagents include lignosulfonates, citric acid, EDTA, and borax. Otherretarding materials include phosphonates, such as those used in scaleinhibition in oil and gas wells and also in water treatment processesfor boilers, cooling towers, etc. Examples of such materials are thosemarketed by Monsanto Company under the trade name "DEQUEST" Specificexamples are DEQUEST 2000, 2006, 2010, 2016, 2060, and 2066.

Other retarding materials include some phosphates such as sodium,potassium, calcium or magnesium glycerophosphates, borates such as boricacid and its salts, salts of organic acids such as sodium or potassiumgluconate, sodium or potassium glucoheponate and sodium citrate. Organicamines can also be retarders.

Combinations of borax, boric acid or other borate salts and some borateester surfactants such as monoethanolamine borate with lignosulfonate ororganic acid salts are good high temperature retarders. These arecommonly used for high temperature retarders for cements. Salts oforganic polyacids such as EDTA, polyacrylic acid, polymethacrylic acid,itaconic acid, fumaric acid can also retard in some temperature ranges.

Polyalcohols Broadly

Suitable polyalcohols which can be used, if desired, in the universalfluid of this invention include polyols having at least two carbon atomsand two hydroxyl groups but no more than 18 carbon atoms and 13 hydroxylgroups. Nonlimitative examples of such polyalcohols include (carbonchains may be straight chains, branched chains or cyclic) , ethyleneglycol, diethylene glycol, 1,2-propanediol, 1,3-propanediol propyleneglycol, neopentyl glycol, pentaerythritol, 1,6-hexanediol, glycerol,open and cyclic condensation products of glycerol (and otherpolyalcohols) such as diglycerols, triglycerols, tetraglycerols,pentaglycerols, and hexaglycerols, poly(ethylene glycol)s,poly(propylene glycol)s, ethylenepropylene glycol,poly(ethylenepropylene glycol)s, ethylenepropylene glycol copolymers andethylenebutylene glycol copolymers and ethylenebutylene glycolcopolymers, 1,5,6,9-decanetetrol, 1,1,4,4-cyclohexanetetramethanol,1,2,4,5-cyclohexanetetramethanol, 1,4-cyclohexanedimethanol,1,3-cyclopentanedimethanol, 1,2,4,7-heptanetetrol,1,2,3,5-heptanetetrol, 1,5,8-nonanetriol, 1,5,9-nonanetriol,1,3,5,9-nonanetetrol, 1,3,5-heptanetriol, 2,4,6-heptanetriol,4,4-dimethyl-1,2,3-pentanetriol, 1,1,3-cyclohexanetrimethanol,1,3,4-cycloheptanetriol, 1,1-cyclopropanediol, 1,2-cyclopropanediol,1,2,3-cyclopropanetriol, 1,1-cyclopropanedimethanol,1,2-cyclopropanedimethanol, 1,2,3-cyclopropanetrimethanol,1,1-cyclobutanediol, 1,2-cyclobutanediol, 1,3-cyclobutanediol,1,2-cyclobutanedimethanol 1,2,3-cyclobutanetriol,1,2,4-cyclobutanetriol, 1,2,3,4-cyclobutanetetrol,1,3-dimethyl-1,2,3,4-cyclobutanetetrol, 1 hydroxy cyclobutanemethanol,2-methyl-1,2-butanediol, 2-methyl-1,2-butanediol,3-methyl-2,2-butanediol, 1,2-pentanediol, 1,3-pentadiol,1,4-pentanediol, 2,3-pentanediol, 2,4-pentanediol, 1,2,3-pentanetriol,1,2,4-pentanetriol, 2,3,4-pentanetriol, 1,1-cyclopentanediol,1,2-cyclopentanediol, 1,3-cyclopentanediol, 1,2,3-cyclopentanetriol,1,2-hexanediol, 1,3-hexanediol, 1,2,3-hexanetriol, 1,2,4-hexanetriol,1,2,3,4-hexanetetrol, 1,1-cyclohexanediol, 1,2-cyclohexanediol,1,4-cyclohexanediol, 1,2,4-cyclohexanetriol, 1,2,5-cyclohexanetriol,1,2,3,4-cyclohexanetetrol, and 1,2,3,5-cyclohexanetetrol.

Cyclic Polyol Production

A general chemical composition formula, disregarding the order,representative of another class of polyols is as follows: ##STR1## wherex≧1 and y≧0.

As a specific example, x=2 and y=5.

Polyethercyclicpolyols having a ratio of x:y within the range of 5:2 to1:10 represent a presently preferred class of soluble polyols. Solublepolyetherpolyols with a relatively high molecular weight (in excess of50,000) are also preferred in some instances. Also of significance isthe ether-to-alcohol ratio is given by the following formula (forpolyetherpolyols produced from glycerol): ##EQU1## where e=the ratio,x=the number of rings and y=the number of glycerol moleculesincorporated into the molecule that are not in the ring structure. Ifx=0 then the ratio varies from 0.25 to a value approaching 1.Preferably, x is not 0, and thus the polyol is a cyclic polyetherpolyol.A typical e range for such polyalcohols is 1.4 to 1.8. The sum of 2x+yyields the number of glycerol monomers constituting the polymer.

A more complete description of these polyethercyclicpolyols is found inHale and Cowan, U.S. Pat. No. 5,058,679 (Oct. 22, 1991), the disclosureof which is incorporated herein by reference.

Broadly, the reaction involves heating a polyol with at least threehydroxyl groups, at least two of which are adjacent each other, withremoval of water to form a condensed product.

With glycerol as the primary reaction medium, it is preferable to removeat least 1.05 and more preferably, at least approximately 1.12 moles,but no more than 1.19 moles of water per mole of glycerol. Mostpreferably, 1.12 to 1.15 moles of water per mole of glycerol in theproduct should be removed. If the feed contains an appreciable amount ofpredehydrated glycerol polymers, then the remaining dehydration will beless than 1.2 moles per mole of glycerol. As an example, for a knowncommercial product which typically contains 15 percent by weight ofbis(hydroxymethyl)dioxanes, and 85 percent by weight of glycerol, thedehydration can be calculated as follows. For approximately 100 grams offeed there are 85 grams of glycerol (0.92 moles) and 15 grams ofbis(hydroxymethyl)-dioxane (0.1014 moles). The glycerol component willhave to lose 0.92×1.2=1.104 moles of water. The 0.1014 moles ofbis(hydroxymethyl)-dioxane is derived from 0.2028 moles of glycerol byremoval of 0.2028 moles of water; 1.2 total moles water per mole ofglycerol should be removed, i.e., 0.2028×1.2=0.2434 moles. Thus it isnecessary to remove 0.2434-0.2028=0.0406 moles of water. The total to beremoved is 1.104 moles from the glycerol+0.0406 moles from thebis(hydroxymethyl)-dioxane=1.1496 moles water (or approximately 1.15moles) from the 100 grams of the known commercial product.

Therefore, it is necessary to remove close to 1.2 moles of water foreach mole of glycerol which enters into the condensation of an initiallypartially dehydrated glycerol feed stream. Alternatively, in most casesinvolving complex feed streams, it would be appropriate to carry out thereaction and select a final maximum reaction temperature at set pressureconditions, such as is known from previous experience to yieldsatisfactory polyethercyclicpolyol preparations.

The solubility in water is a function of molecular weight, the presenceor absence of long chain dihydric alcohol units, and ether units, witheither higher molecular weight, long chain dihydric alcohols or higherether ratios, or all three, giving insoluble polyols. Suitable dihydricalcohols for copolymerization with the alcohols having at least threehydroxy groups include those of the formula ##STR2## wherein R' is a1-20 preferably 2-10 carbon atom hydrocarbyl group. Non-condensedpolyalcohols of sufficiently high molecular weight can also be insolubleand such insoluble alcohols can also encompass monohydroxy alcohols.Thus, the soluble alcohol must be polyhydric and the insoluble can bepolyhydric or monohydric although polyhydric is preferred.

Epoxy-Containing Cyclic Polyetheralcohols

In some instances it may be desirable to use a polyol containingparaffinic and/or aromatic groups linked by ether linkages to the polyolstructure, these ether linkages having their origin in glycidyl ether orepoxy groups. Such polyols can be produced as disclosed in said Hale andCowan patent for polyethercyclicpolyols generally except an epoxy resinis incorporated by reaction. Specifically, they can be produced by:

(a) heating a reaction mixture comprising a reactant selected from thegroup consisting of (1) a polyol having at least three hydroxyl groupsof which at least two of the hydroxyl groups are vicinal, (2) precursorsof the polyol, (3) cyclic derivatives of the polyol, and (4) mixturesthereof, said heating initiating the thermal condensation:

(b) removing water formed during the thermal condensation; and

(c) prior to the condensation going to completion, admixing an epoxyresin with the reaction mixture.

Epoxy resins are characterized by the presence of a three-memberedcyclic ether group commonly referred to as an epoxy group, 1,2-epoxide,or oxirane. Preferred epoxy resins are diglycidyl ethers, triglycidylethers, tetraglycidyl ethers, as well as multifunctional glycidyl etherswith more than four epoxy rings which, in the reacting glycerol orpolyol medium, result in the formation of higher molecular weightpolyethercyclicpolyols with substantially improved properties inconnection with drilling fluid performance.

A particularly useful epoxy is a difunctional glycidyl ether such as"EPON 828" (a trademark of Shell Oil Company) which significantlyincreases the number average (M_(n)) molecular weight of a significantfraction of the polyethercyclicpolyols preparation. While not wishing tobe bound by theory, addition of 3 weight percent of "EPON 828" couldresult in doubling the molecular weight of between 10 and 20 percent ofthe preparation. By thus increasing the molecular weight of asignificant portion of the preparation, the copolymerization of "EPON828" results in significantly boosting the M_(w) value of the sample,with attendant significant improvements in the performance of theresulting polyethercyclicpolyols polymer/polyethercyclicpolyolsdiglycidyl ether copolymer mixture.

In order to obtain polyethercyclicpolyols with maximum coveragepotential, it is suitable to use tri- and tetraglycidyl ethers whichwill direct polymerization along more than one direction in a planarconfiguration. It is theorized, although applicant does not wish to belimited to this theory, that the use of such epoxies facilitates thecoverage of openings in the clay surface of an oil well through whichwater can enter the clay. While not wishing to be bound by theory,applicant believes that molecules which are substantially planar instructure are most useful with the invention when it is employed as partof a drilling fluid additive. Additionally, the attachment of severalpolyethercyclicpolyols onto the same central molecule of polyglycidylether, allows multiple coordination of cationic species to occur throughthe electron donating oxygen atoms in the ether linkages, which resultsin formation of large molecular aggregates that can inhibit themigration of water molecules from the aqueous phase of the water-baseddrilling mud onto the hydrophilic clay solids of the formation.Water-based drilling muds containing polyethercyclicpolyols actessentially in a manner similar to that of oil-based muds. This theory,of course, is not limiting of the application of these materials in thisinvention.

Experimental results have shown that the impact of using multifunctionalglycidyl ethers on the values of M_(w) and on the performance,particularly as regards swelling of clays when the epoxy-containingpolyols are used as a drilling fluid additive, is truly significant.Thus, when using 3 weight percent "EPON 828" and 3 weight percent "EPON1031" (trade names of Shell Oil Company), the M_(w) values arecorrespondingly 78,015 and 151,000, and the swelling inhibition is thehighest with "EPON 1031", with good performance also observed oninhibition of fluid loss and dispersion.

Although the observation of bimodal distribution of molecular weights inGPC (three-column chromatography) does not require the presence ofepoxies, nevertheless, incorporation of epoxies into thepolyethercyclicpolyols structure has a significant impact on therelative amounts of "large" molecules in the polyethercyclicpolyolproduction, increasing the ratio of large volume molecules/small volumemolecules.

The following epoxies are considered useful in the present invention:##STR3##

With respect to "EPON 828", it is preferred to add the material in aplurality of aliquots, generally two or three aliquots, generallyobserving the layered addition until about 40 to 50 percent of thereaction is complete.

With "EPON 1031", it is suitable to add all the material at thebeginning of the condensation reaction. It is suitable to add a largeramount of "EPON 828" than of "EPON 1031" because of the lower molecularweight of "EPON 828". It is preferred to avoid adding "EPON 828" attemperatures in excess of 270° C., due to risk of prematurepolymerization. The addition of more volatile polyglycidyl ethers mustbe carried out with caution due to their potential toxicity andrelatively higher volatilities.

The epoxy resin can be used in an amount sufficient to give 0.5 to 5weight percent material from the epoxy resin incorporated in the epoxypolyethercyclicpolyol. Alternatively, a relatively high epoxy contentcan be utilized, say 6 to 67, preferably 15 to 40 wt %. Thus, viewed interms of the polyol, the epoxy component content can vary from 0 to 67wt % based on the total weight of the polyol. In utilities where shalestabilization is the primary consideration, high epoxy content ispreferred. In other instances, low epoxy content may be preferred.

The initial pressure can be higher when making the higher epoxy materialas compared with the initial pressure preferred for the low epoxy. Forinstance, the initial pressure can be greater than 180 torr. Generally,the initial pressure will be between 250 and 500, preferably 250-350torr, when the starting polyhydric alcohol component is glycerine. Theinitial temperature is generally between about 175° and 275° C.,preferably between 200° and 260° C., more preferably between 210° and250° C. If desired, the reaction can be terminated before 1.07 moles ofwater per mole of polyol are removed. Preferably, the polyhydric alcoholmonomer is introduced into the reaction zone in a single addition andthe epoxy introduced in a plurality of additions, preferably 2 to 10,most preferably 3 to 6 when utilizing the higher epoxy. Generally, ifhigher epoxy content materials are being produced, more additions areutilized and the addition of the epoxy could be continuous. With thehigh epoxy content materials diglycidyl ethers are preferred instead ofthe tri- and tetraglycidyl ethers.

During the course of the reaction the temperature with the preferredpolyol, glycerol, is generally increased to a range of 250°-300° C.,preferably 251°-280° C., more preferably 260°-273° C. at essentially theinitial pressure. Thereafter, the temperature is increased to 260°-310°C., preferably 261-300, more preferably 280°-287° C. at a pressure ofless than 180, more preferably 40-130mm Hg.

References herein to pressure in terms of torr or mm of Hg refers toabsolute pressure, i.e., anything under 760 torr represents a partialvacuum.

Ethoxylated Propoxylated Alcohols

Another preferred class of the polyalcohols is ethoxylated propoxylatedalcohols of the following general formula

    R[(EO).sub.m -(PO).sub.n ].sub.z OH

where

EO=an ethoxy unit

PO=a propoxy unit

R=an alkyl chain of 2-16 carbon atoms, preferably 3-16, most preferably4-10 carbon atoms.

At least one of m or n is greater than 0. In these EO/PO copolymers, mand n are variable and the sum of m plus n determines their numberaverage molecular weight, which ranges from 500 to 15,000, preferablyfrom 600 to 10,000.

The m/n ratio determines the hydrophobic/hydrophilic balance (HLB),water solubility and nonionic surfactant properties of the copolymer.Solubility depends on the ionic strength of the aqueous medium (higherionic strength results in lower solubility) and temperature. Thetemperature relationship is in a sense an inverse relationship since athigher temperatures the polyalcohols exhibit a cloud point and at lowertemperatures they go back into solution. High temperatures in somesystems have the same effect as high salt solutions.

Polyalcohol Function

Soluble polyalcohols, if used, work in a similar way to the salt in thatthey lower water activity and bind to the clay so as to reduce theamount of hydration. In addition high molecular weight solublepolyalcohols may reduce the communication between the formation porepressure and the hydrostatic pressure due to mud weight. Both mechanismsare advantageous in terms of cuttings stability and actual wellborestability. Thus, the polyalcohol acts in combination with the salt togive particularly good shale stabilization.

While Applicants do not wish to be bound by theory, it is believed theinsoluble (essentially insoluble, there is some wetting of the alcohol)alcohols function primarily by a plugging action whereby they preventmud weight/formation pore pressure communication. This is not just asimple mechanical plugging, however. The alcohol groups bind to the claystructure. The combination of soluble and insoluble alcohols results inbetter wetting of the insoluble alcohols and thus better delivery to thepore throats. This is independent of the soluble alcohols beneficialfunctions previously discussed. Also, in systems having both salt andpolyalcohol, the benefits of both are still obtained.

If the pore throats are large enough, the soluble alcohol will bind tothe clay particles in the shale and reduce hydration. The same reactionwill be the reason that the polyalcohol will enable high solidstolerance in the drilling fluid. Binding and coating the solids (clays,formation solids and other loose drilling fluid solids) will lower theparticle to particle interaction which will lower the viscosity.

Again, while not wishing to be bound by theory, applicant believeswellbore stability is enhanced, when polyalcohols are used, becausethese materials form micelles which micelles plug pore entrances.

Thus, density alone does not determine the effectiveness of a drillingfluid in maintaining sufficient pressure to counterbalance the formationpressure. There is also a chemical effect. The cuttings/wellbore (shale)stabilizers described herein, particularly the high molecular weightsoluble polyalcohols, insoluble polyalcohols andethoxylated/propoxylated polyalcohols may lower mud weight pore pressurecommunication so the mean effective stress is not lowered in theformation. Thus pore pressure is not increased and thus boreholeinstability is reduced. While not wishing to be bound by theory, it isbelieved the soluble alcohol in solution binds water in the drillingfluid which lowers the water activity of the drilling fluid, i.e.,lowers the molar free energy of the solution. The resulting molar freeenergy of the water in the drilling fluid compared with that in theformation results in a chemical stabilization of the wellbore by notincreasing the formation pore pressure or possibly lowers the porepressure, thus keeping the stress state of the wellbore such that itdoes not exceed the strength of the formation.

If only one polyalcohol is used it is preferably either a solublepolyalcohol with a relatively high weight average (M_(w)) molecularweight in excess of 50,000 or else a low molecular weight insolublealcohol having a molecular weight of less than 10,000.

Alternatively, a mixture of a soluble and an insoluble alcohol can beused. Particularly preferred are mixtures of (a) a cyclicpolyetherpolyol which is water soluble with (b) a cyclic or acyclicpolyetherpolyol which is insoluble.

The soluble polyalcohols and mixtures thereof have been found to improveinterfacial sealing. The insoluble alcohol can be any alcohol broadly,preferably a polyol, or specifically a polyethercyclicpolyol. Preferredinsoluble polyols include polyglycols such as a high molecular weightpoly(propylene glycol) with sufficient hydroxyl groups converted toether groups to be insoluble.

The polyalcohols, particularly the polyethercyclicpolyols, perform anumber of separate functions in addition to retardation. They functionin the drilling fluid as a shale stabilizer and fluid loss additive inaddition to retarding the setting of the slag during drilling and thenin the cementitious slurry they serve as a rheology control agentreducing the increase in viscosity of the cementitious slurry thusmaking it more pumpable. While not wishing to be bound by theory,applicants believe the fluid loss prevention comes about indirectly. Thepolyalcohol has a wetting capability and absorbs onto polymers and/orclay in the drilling fluid as noted above and perhaps otherwise modifiesthe drilling fluid, thus improving the fluid loss capability of theadditives in the system. The polyalcohols also tend to improve bondingof the cement to the casing or liner and to the wellbore.

Fluid Loss Additive

A fluid loss additive is generally selected from synthetic polymers suchas biopolymers, starch, clay, and, as noted hereinabove, polyalcoholssuch as cyclicpolyetherpolyol (which probably acts primarily as anenhancer for the others). Carboxymethyl cellulose can also be usedalthough because of the presence of salt, a higher concentration isneeded (as compared with other polymers) to be effective.

Clay, when used, is generally present in an amount within the range of 2to 50, preferably 5 to 30, more preferably 10 to 20 lbs/bbl of drillingfluid.

Rheology Control

A rheology control agent in the context of a drilling fluid keeps solidsfrom settling out and may be viewed as a viscosifier. The preferredviscosifier for the universal drilling fluid include biopolymers andclays. The silicates also function as viscosifiers as well as boreholestabilizers. Starch can also function, generally in a secondary role, asa viscosifier. The starch, for instance, when used, is generally used inan amount within the range of 2 to 15, preferably 5 to 10 lbs/bbl ofdrilling fluid. The biopolymers can be used in an amount within therange of about 0.1 to about 3, preferably 0.2 to 1 lbs/bbl of drillingfluid. All of the components, if used, are used in an amount effectiveto produce the desired effect, i.e., in this case, a sufficient increasein viscosity so that the mud will carry cuttings up out of the borehole.

Mixed Metal Hydroxide

Mixed metal hydroxides can be used in the drilling fluid to impartthixotropic properties. In such instances, a thinner such as alignosulfonate is preferably added before adding polymer. The mixedmetal hydroxides provide better solids suspension. This, in combinationwith the settable filter cake provided in the technique of thisinvention greatly enhances the cementing in a restricted annulus, forinstance.

The mixed metal hydroxides are particularly effective in muds containingclay such as sodium bentonite in addition to the metal oxide. Preferredsystems thickened in this way contain from 1-20 lbs/bbl of clay such asbentonite, preferably 2-15lbs/bbl, most preferably 7 to 12 lbs/bbl. Themixed metal hydroxides are generally present in an amount within therange of 0.1 to 2 lbs/bbl of drilling fluid, preferably 0.1 to 1.5lbs/bbl, most preferably 0.7 to 1.2 lbs/bbl. A more detailed descriptionof mixed metal hydroxides can be found in Burba, U.S. Pat. No. 4,664,843(May 12, 1987).

Weight Material

The universal drilling fluid must be formulated to have the weightrequired for a particular drilling operation being conducted. This iswell known in the art and in many instances sufficient weight may beprovided by the coal slag. Optionally, however, the weight can beadjusted using conventional weighting agents such as barite (bariumsulfate). The amount, if any, used would be the amount necessary to givethe desired mud density. Other suitable weighting agents includetitanium oxides such as TiO₂ and iron oxides such as hematite andillmenite.

Shale Stabilizer

This is an optional ingredient. In some formations, the zones beingdrilled do not require stabilization. Also, the silicate, if present,functions as a shale stabilizer as does lime, if present. Again, thepolyalcohol, if present, serves as a shale stabilizer in addition to aretarder. Finally, as noted above, in systems where the aqueous mediumcontains a salt, the salt acts as a shale stabilizer.

Deflocculant

A deflocculant such as a carbohydrate polymer can be used if needed.Generally, deflocculants, if present, will be used in an amount withinthe range of 0.5 to 10 lbs/bbl of drilling fluid.

Dual Functionality Detail

The following Table summarizes these dual functionality concepts.

                  TABLE A                                                         ______________________________________                                        Function                                                                      Drilling Fluid       Cementitious Slurry                                      Additive                                                                              Primary    Secondary Primary Secondary                                ______________________________________                                        Water   Cuttings             Hydrating                                                Carrier              Agent                                            Synthetic                                                                             Fluid loss           Fluid loss                                       polymer.sup.1                                                                         control              control                                          Starch.sup.2                                                                          Fluid loss Viscosity Fluid loss                                                                            Viscosity                                        control              control                                          Bio-    Viscosity  Fluid Loss                                                                              Viscosity                                                                             Retarder                                 polymer.sup.3                                                                 Silicate                                                                              Viscosity  Shale     Acceler-                                                                               --                                                         stabilizer                                                                              ator                                             Carbox- Deflocculant                                                                             Retarder  Defloccu-                                                                             Retarder                                 ylate                        lant                                             polymer.sup.4                                                                 Barite.sup.5                                                                          Density     --       Density Solids for                                                                    compressive                                                                   strength                                 Bentonite.sup.6                                                                       Viscosity  Fluid loss                                                                              Fluid loss                                                                            Solids for                                       (solids    control   control compressive                                      suspension)                  strength                                 Clay/    --         --       Solids   --                                      Quartz                                                                        dust.sup.7                                                                    Slag.sup.8                                                                            Cementitious                                                                             Weight    Cemen-  Solids                                                      cuttings  titious                                                             stabilizer                                                 Lime.sup.9                                                                            Shale      Alkalinity                                                                              Acceler-                                                                              Solids                                           stabilizer           ator                                             PECP.sup.10                                                                           Shale      Enhance   Rheology                                                                              Bond                                     polyalcohol                                                                           stabilizer fluid loss                                                                              control Improver/                                        retarder   properties        Retarder                                 NaCl    Shale       --       Strength                                                                               --                                              stabilizer           enhance-                                                                      ment                                             ______________________________________                                         .sup.1 A synthetic polymer manufactured by SKW Chemicals Inc. under the       trade name "POLYDRILL", for instance.                                         .sup.2 Starch made by Milpark Inc. under the trade name "PERMALOSE", for      instance.                                                                     .sup.3 "BIOZAN", a biopolymer made by Kelco Oil Field Group, Inc., for        instance. This is a welan gum and is described in U.S. 4,342,866.             .sup.4 A watersoluble carbohydrate polymer manufactured by Grain              Processing Co. under trade name "MORREX", for instance.                       .sup.5 Barite is BaSO.sub.4, a drilling fluid weighting agent.                .sup.6 Bentonite is clay or colloidal clay thickening agent.                  .sup.7 A clay/quartz solid dust manufactured by Mil White Corp. under the     trade name "REVDUST", for instance.                                           .sup.8 Coal slag manufactured by Fairmont Minerals under the trade name       "BLACK MAGNUM" is suitable.                                                   .sup.9 CaO                                                                    .sup.10 Polycyclicpolyetherpolyol                                        

Functionally, there are three essential and six optional ingredients inan ideal drilling fluid, namely,

1) a cuttings carrier,

2) a latent cementitious material

3) drill solids,

4) generally, a rheology control agent (viscosifier),

5) generally a fluid loss additive,

6) (optional) a shale stabilizer,

7) (optional) a weighting agent,

8) (optional) a deflocculant,

9) (optional) a retarder.

In functional terms, there are two essential ingredients and severaloptional ingredients in a cementitious slurry, namely,

1) the cementitious or hydratable material itself,

2) hydrating agent,

3) (Optional) accelerator (essential but may be provided by action ofheat and/or the silicate, if present, in the drilling fluid). Otheraccelerators may be added to the cementitious slurry if desired.

4) (Optional) rheology control/thinner (as needed).

5) (Optional) density agent (as needed).

6) (Optional) fluid loss control (will generally be carried over fromthe drilling fluid),

7) (Optional) a shale stabilizer (as needed). Will generally be carriedover from the drilling fluid,

8) (Optional) solids or aggregate, and

9) (Optional) strength enhancing agents.

As can be seen from the above Table, a composition as simple as onehaving water, coal slag, and the drill solids provides all of theessential drilling fluid functions and cementitious functions. This isbecause the water serves as the cuttings carrier in the drilling fluidand the hydrating agent in the cementitious slurry. The coal slag servesa two-fold function by providing the essential cementitious function andthe optional weight function in the drilling fluid and serves the dualfunction of providing the cementitious material in the cementitiousslurry. The coal slag can also serve as a shale stabilizer in somesystems. A polyalcohol, if present, serves the four-fold function in thedrilling fluid of retardation of the hydration of the blast furnace slagduring drilling, modification of the water activity of the aqueouscontinuous phase (shale stabilization), reduction of mud weight/porepressure communication and fluid loss control in the drilling fluid andthen serves the dual function as a rheology control agent in thecementitious slurry to give the desired thinning. The polyalcohol alsoserves in the final cement as a bond improver. A silicate, if present,serves the two-fold function in the drilling fluid of viscosifier andmodifier of the water activity of the aqueous continuous phase (shalestabilization) in the drilling fluid and then serves the dual functionas an accelerator in the cementitious slurry.

Preferably, the universal fluid composition further contains a salt suchas sodium chloride to provide further shale stabilization and strengthenhancement in the cementitious slurry. Similarly, the compositionpreferably contains a secondary fluid loss control agent such as asulfonated synthetic polymer or starch (which also gives viscosity).Also, it may be preferred in many instances to use a biopolymer to giveviscosity. Similarly, lime may be present to further givecuttings/wellbore stabilization and to provide alkalinity in thedrilling fluid and to function as an accelerator and solids in thecementitious slurry.

Almost always a clay such as bentonite or prehydrated bentonite will bepresent which provides fluid loss control in the drilling fluid and inthe cementitious slurry and also solids in the cementitious slurry. Insome instances, however, drilling fluids which are initially clay-free(free of prehydrated bentonite, for instance) are preferred. The clay isfrequently included in the initial drilling fluid and in any event willalmost always be encountered during drilling. Frequently, clay,prehydrated in fresh water, is used initially to give the functions suchas fluid loss control since the clay in a salt water environment doesnot hydrate readily and thus imparts less viscosity to the fluid. Thisis generally referred to as the "yield", i.e., the amount of viscosityimparted by the clay.

If flocculation is a problem in a particular system, a deflocculant suchas a carbohydrate polymer, acrylate polymer, sulfonate polymer, styrenemaleic anhydride polymer, organic acid or polyalcohol can be used whichwill also provide retardation of the coal slag during drilling and actas a deflocculant in a cementitious slurry.

Finally, if additional weight is desired in the drilling fluid, this canbe provided with a weighting agent such as barite, hematite, illmenite,titanium dioxide, manganese oxides or bentonite which will give solidsand weight to the cementitious slurry.

The clay/quartz dust is shown in Table A and was used in laboratorytests of drilling fluids to simulate drill solids produced in actualdrilling, and thus would not generally be added to an actual drillingfluid.

While the ingredients can be added in any order, there are twocombinations where a significant improvement flows from the additionsequence. First, if a biopolymer and lime are used, the biopolymershould be added after the lime. This gives better yield, i.e., enhancesviscosity imparted and fluid loss prevention capabilities. Second,polymers such as starch and biopolymers should be added before thepolyalcohol to allow the polymers to hydrate before contact with thepolyalcohol. This gives better hydration and thus better polymerextension (swelling).

Unlike Portland cement which is incompatible with ingredients which aregenerally in drilling fluids, coal slag is compatible with drillingfluids. Thus, coal slag can simply be added to any conventional drillingmud to give a drilling composition which can thereafter be set throughthe action of an activator as in this invention. Such systems areentirely operable and represent an advance in the art. The use of a limebased system provides the optimum base fluid for implementation of coalslag in a universal fluid. Silicate muds also give advantages with coalslag similar to those obtained with lime muds.

The drilling can be carried out for any period of time desired with thesame drilling fluid (augmented with fresh fluid to compensate for lossand greater hole depth) without the coal slag setting. For instance, thedrilling can be carried out for greater than one day up to any timeneeded, generally between one day and 100 days. Then, on activation, thecoal slag will set to give its full potential of compressive strength.

Activation

In its simplest form, activation may occur simply through an increase intemperature and/or the effect of lime or residual silicate, if present.For instance, during drilling the circulation of the drilling fluid hasas one of its functions the carrying off of generated heat and any heatfrom the formation. Thus, during the drilling the retarders, if any, inthe drilling fluid inhibit hydration of the coal slag as does thecirculation itself. On cessation of drilling and displacement of thedrilling fluids selectively into the area to be cemented, thetemperature will rise in those systems where the circulation wascarrying off heat from the formation to a degree sufficient to set thecementitious slurry.

Generally, no retarder is required. Furthermore, because the coal slagis able to function for a long time in a drilling fluid without settingand then set in a controlled amount of time after adding an activator,the universal fluid of this invention is ideally suited for operation inhigh temperature zones, i.e., zones where the formation temperature isabove 150° F., or even above 200° F. Thus, for example, duringoperations at 150°-250° F. wellbore temperatures, drilling can becarried out using the universal fluid of this invention and then thecasings and liners can be cemented with a coal slag cementitious slurryof this invention.

In most instances, however, an activator system will be added to thedrilling fluid between the drilling operation and the cementingoperation. The activator system can be simply additional coal slag and,in any event, additional coal slag will generally be incorporated. Also,other ingredients which are, or which may be, present in the drillingfluid and which have an accelerator function can be added in additionalquantities between the drilling and the cementing. For instance,silicate can be added or if it has lime, i.e., is a lime mud, additionallime can be added.

Suitable activators which are generally not a part of the drillingfluid, but are added between the drilling operation and the cementingoperation, include lithium hydroxide, lithium carbonate, sodiumsilicate, sodium fluoride, sodium silicofluoride, magnesium hydroxide,magnesium oxide, magnesium silicofluoride, zinc carbonate, zincsilicofluoride, zinc oxide, sodium carbonate, sodium bicarbonate,titanium carbonate, potassium carbonate, potassium bicarbonate, sodiumhydroxide, potassium hydroxide, potassium sulfate, potassium nitrite,potassium nitrate, sodium or potassium aluminate, calcium hydroxide,sodium sulfate, copper sulfate, calcium oxide, calcium sulfate, calciumnitrate, calcium nitrite, and mixtures thereof.

Calcium oxide or calcium hydroxide (lime) is preferred as the activatoror at least as a part of the activator. The preference for using atleast some calcium oxide or lime is particularly strong in coal slagshaving a low ratio of lime to silica such as those slags from highercarbon content coals. A mixture of caustic soda (sodium hydroxide) andsoda ash (sodium carbonate) can also be used, either alone or incombination with calcium oxide. When mixtures of alkaline agents such ascaustic soda and soda ash are used the ratio can vary rather widelysince each will function as an accelerator alone. Preferably, about 1 to20 lbs/bbl of caustic soda, more preferably 2 to 6 lbs/bbl of causticsoda are used in conjunction with from 2 to 50 lbs/bbl, preferably 2 to20 lbs/bbl of soda ash. Generally, 2 to 70 lbs/bbl of total activator(other than calcium oxide) is used. The references to "lbs/bbl" meanspounds per barrel of final cementitious slurry. When used alone, thecalcium oxide is generally used in an amount within the range of 5 to300 lbs/bbl, preferably 20 to 175 lbs/bbl. When used with otheraccelerators, the amount is generally within the range of 5 to 200lbs/bbl, preferably 20 to 125 lbs/bbl. Lime or calcium hydroxide wouldbe used in an amount to give the indicated amount of calcium oxide. Whencalcium oxide is used, the total activator can be greater than the 2-70lbs/bbl range by the amount of calcium oxide used. As a generalproposition, if the CaO/SiO₂ weight ratio of the coal slag is less thanabout 0.15:1, additional lime will generally be added so as to reach aratio of at least 0.20:1, preferably at least 0.24:1, and in someinstances, lime will be added to give a ratio of at least 0.5:1.Generally, lime is added to give a ratio within the range of 0.2:1 to2.0:1.

In some instances, it may be desirable to use a material for aparticular effect along with the activator even though it may also actas a retarder. For instance, a chromium lignosulfonate may be used as athinner in the cementitious slurry along with the activator even thoughit also functions as a retarder.

Other suitable thinners include chrome-free lignosulfonate, lignite,sulfonated lignite, sulfonated styrene maleic-anhydride, sulfomethylatedhumic acid, naphthalene sulfonate, a blend of polyacrylate andpolymethacrylate, an acrylamideacrylic acid copolymer, phenol sulfonate,dodecylbenzene sulfonate, sulfomethylated tree extract, stearyl amineand lauryl amine surfactants, sulfonated styrene-toluene copolymers, andmixtures thereof.

The drilling process is carried out as described hereinabove with theuniversal fluid to produce a borehole through a plurality of strata,thus laying down a filter cake. Because the filter cake comprises coalslag, it will eventually hydrate with time to produce a solid. Thishydration is further accelerated by migration of activators from thecementitious slurry when it is displaced into the annulus between a pipeand the borehole wall on which the filter cake is laid down.Furthermore, because the cementitious slurry is compatible with thedrilling fluid, a good bond will be obtained between the cementitiousslurry and the borehole wall. Further, the cementitious slurry iscompatible with any mud which is not removed during displacement. Thus:

a) drilling filter cake deposited while drilling through permeable zoneswill be converted into an effective sealant;

b) whole mud that has not been removed from washed out sections of thehole during displacement will harden with time and, therefore, providean effective sealant and lateral support to the casing.

Filter Cake Setting

In yet another embodiment using the composition of this invention thedrilling process is carried out as described hereinabove with theuniversal fluid to produce the borehole through a plurality of stratathus laying down the filter cake. Prior to the cementing operation, anactivator is passed into contact with the filter cake, for instance bydisplacing out the drilling fluid and circulating a fluid containing theactivator down the drill string and up the annulus between the drillstring and the filter cake, or else the drill string is removed and thecasing inserted and the activator circulated down the casing and up theannulus (or down the annulus and up the drill string or casing).Preferably, the circulation is carried out by using the drill string,this being one benefit of this embodiment of the invention whereby thefilter cake can be "set" to shut off gas zones, water loss, or to shutoff lost circulation in order to keep drilling without having to removethe drill string and set another string of casing. Alternatively, theactivator can be added to the drilling fluid instead of using a separatefluid. This filter cake setting can also be used to stabilize zoneswhich may be easily washed-out (salt zones wherein the salt is solublein water, for instance) or other unstable zones. After the drilling iscomplete, the drill string is removed, and the cementing carried out.

Conventional spacers my be used in the above described sequence. Also,any leftover fluid having activators therein may be displaced out of theborehole by the next fluid and/or a spacer fluid and stored forsubsequent use or disposal.

In this embodiment where the filter cake is "set", the activator can beany of the alkaline activators referred to hereinabove such as a mixtureof sodium hydroxide and sodium carbonate.

Lime

As previously noted, preferred systems contain lime. These systems thusare analogous to conventional drilling fluids known as high lime, lowlime, and low lime/salt/alcohol. By "low lime" is meant a drilling fluidhaving about 0.5 to 3, generally 0.5 to 2.0 lbs of unreacted lime perbarrel of drilling fluid. By "high lime" is meant a drilling fluidhaving from greater than 3.0 to 15 lbs of unreacted lime per barrel ofdrilling fluid. The low lime/salt/alcohol fluids have about 1 to 4.0lbs/bbl of unreacted lime, about 18 to 109 lbs/bbl of salt such assodium chloride, and about 1 to 168 lbs/bbl, preferably 10 to 80lbs/bbl, more preferably 15 to 65 lbs/bbl, most preferably 40 to 60lbs/bbl of a polyhydric alcohol per barrel of drilling fluid. Oneparticularly advantageous aspect of this invention is the apparenttendency for lime in the coal slag to be solubilized during drilling, orjust during mixing, thus converting a non-lime drilling fluid into alime mud. Suitable non-lime muds include seawater/lignosulfonate muds,seawater/gypsum muds and sodium chloride/partially hydrolyzed acrylamidemuds.

Ingredient Ratios

Coal slag is present in the drilling fluid (universal fluid) in anamount within the range of about 1 to about 100 lbs/bbl of finaldrilling fluid, preferably 10 to 80 lbs/bbl, most preferably 20 to 50lbs/bbl. As noted above, additional coal slag is generally added betweenthe drilling operation and the cementing operation to give a totalconcentration of coal slag in the cementitious slurry within the rangeof from about 20 to 600 lbs/bbl, preferably 100 to 500 lbs/bbl, mostpreferably 150 to 350 lbs/bbl.

The silicate, if present, is generally present in an amount within therange 1 to 100, preferably 2 to 15, most preferably 5 to 10 lbs/bblbased on barrels of drilling fluid.

The concentration of the polyalcohol, if present, in the water phase ofthe universal fluid of this invention will generally be 1 to 50% byvolume and preferably from about 3 to 30% by volume based on the volumeof water, more preferably from 5 to 25% by volume, most preferablybetween 10 and 20% by volume. The soluble:insoluble polyols aregenerally used in weight ratios of about 0.1:1 to about 10:1, preferably0.25:1 to 2:1, more preferably 0.5:1 to 1:1 soluble:insoluble.

Another feature of this invention is the ability to tailor the rheologyof both the drilling fluid and the final cement to the conditions of aparticular wellbore. This results in part from the fact that the use ofcoal slag as the hydraulic material gives a final cementitious slurrywhich is not weakened in the manner that would be the case with Portlandcement if the slurry is more dilute. On the other hand, additional coalslag does not impart extremely high viscosity to the slurry and thus ahigher concentration of hydraulic material can be used if desired.

Dilution

However, in the preferred method of this invention, the drilling fluidis utilized and thereafter diluted prior to or during the addition ofadditional coal slag. The dilution fluid can be the same as the liquidused to make the drilling fluid or it can be different. Generally, itwill be brine, especially if the o drilling fluid was made using brine.It can also be a more concentrated brine. In many instances, it ispreferred that both the dilution fluid and the original liquid used toproduce the initial drilling fluid be seawater. This is especiallybeneficial in offshore drilling applications where fresh water is notreadily available and seawater is.

Thus, a significant improvement in the operating procedure is provided.This is because the density of the drilling fluid can be chosen in thefirst place to be sufficient to avoid inflow into the wellbore becauseof formation pressure but insufficient to rupture the wellbore wall andforce fluid out into the formation. By utilizing the dilution andthereafter the addition of additional blast furnace slag, thecementitious slurry can also have the density tailored to the particularoperation the same as the drilling fluid.

The dilution can be carried out in either of two ways. First, a vesselcontaining drilling fluid can simply be isolated and the desired amountof water or other diluent added thereto. In a preferred embodiment,however, the drilling fluid is passed to a mixing zone as a flowingstream and the diluent added to the flowing stream. Thereafter, theadditional coal slag is added. This avoids highly viscous cementitiousslurry compositions and allows all of the pumping to be done with pipingand pumps associated with the well rig without the need for pumpsdesigned for pumping cement. This is of particular value in the areas too which this invention is of special utility, offshore drilling rigswhere the transportation of additional pumping equipment is particularlyinconvenient. Thus, it is possible to tailor the final density of thecementitious slurry, if desired, to a value within the range of 30% lessto 70% more than the original density of the drilling fluid, preferablywithin the range of 15% less to 50% more, most preferably essentiallythe same, i.e., varying by no more than ±5 weight percent.

Displacement

Conventional displacement techniques can be used to displace theuniversal fluid of this invention with the cementitious slurry. However,because of the greater compatibility of the drilling fluid, especiallylime muds, and the cementitious slurry, wiper plugs and spacer fluidscan be omitted. Thus the cementitious slurry can be placed in directfluid contact with the drilling fluid, especially if it is a lime mud,and the drilling fluid displaced out of the annulus between a pipe beingcemented and a surrounding wall. The cement is, in turn, displaced to apreselected location in the annulus by direct fluid contact with adisplacement fluid such as seawater.

Generally, this involves introducing a cementitious slurry into a casingor liner followed by the displacement fluid and displacing thecementitious slurry down the casing or liner and back up into theannulus surrounding the casing or liner.

EXAMPLE 1

100 lbs/bbl of coal slag sold by Fairmont Minerals under the trade name"BLACK MAGNUM" was added to an aqueous drilling fluid having thefollowing composition:

10 lbs/bbl bentonite gel

20 lbs/bbl drill solids¹

5 lbs/bbl lime

1 lbs/bbl chrome lignosulfonate

The resulting universal fluid was hot rolled at 150° F. for just overone week. The coal slag mixed well with the drilling fluid with noapparent adverse reactions and the drilling fluid remained viable afterthe hot rolling was completed.

EXAMPLE 2

A separate sample of the same coal slag was mixed at a concentration of200 lbs/bbl with an additional sample of the same drilling fluid ofExample 1. 80 lbs/bbl of calcium oxide activator was added and thecomposition set to a hard cement in 72 hours. Comparison of the resultsof these two Examples shows that a coal slag universal fluid can be usedfor drilling for an extended period, even under high temperatureconditions and thereafter on addition of activators and, optionally,more coal slag, converted into a cement.

While this invention has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

What is claimed is:
 1. A method for drilling, comprising:drilling awellbore, with a drill string comprising a drill pipe, utilizing adrilling fluid comprising water, drill solids and coal slag; circulatingsaid drilling fluid down said drill pipe and up an annulus between saiddrill pipe and walls of said borehole, thus laying down a filter cake onsaid walls of said borehole during said drilling and producing a useddrilling fluid.
 2. A method according to claim 1 wherein said drillingfluid comprises, in addition, a polyalcohol component and a silicatecomponent.
 3. A method according to claim 2 wherein said polyalcoholcomponent is a polycyclicpolyetherpolyol of the formula: ##STR4## wherex≧1 and y≧0.
 4. A method according to claim 2 wherein said polyalcoholcomponent is a mixture of a soluble polyol and an insoluble alcohol. 5.A method according to claim 2 wherein said polyalcohol componentcomprises an ethoxylated propoxylated polyol.
 6. A method according toclaim 2 wherein said polyalcohol component comprises an epoxy-containingcyclic polyetherpolyol.
 7. A method according to claim 2 wherein saiddrilling fluid comprises, in addition, a secondary fluid loss additive,a secondary weight material, a secondary shale stabilizer, and acopolymer viscosifer.
 8. A method according to claim 7 wherein saidsecondary fluid loss additive is selected from the group consisting ofsynthetic polymers, starch and bentonite clay, said secondary weightmaterial is barite, said secondary shale stabilizer is lime and saidsilicate is sodium silicate.
 9. A method according to claim 1 whereinsaid drilling fluid comprises, in addition, a viscosifier selected fromthe group consisting of biopolymers, silicates and starch.
 10. A methodaccording to claim 1 wherein said coal slag is water-quenched slag fromthe burning of lignite and has a particle size within the range of 4,000to 9,000 cm² /g.
 11. A method according to claim 1 wherein said drillingfluid is a non-lime mud and said drilling fluid is used for a timesufficient to convert said non-lime mud into a lime mud by dissolvingout lime from said coal slag.
 12. A method according to claim 11 whereinsaid non-lime mud is selected from the group consisting ofseawater/lignosulfonate, seawater/gypsum, and sodium chloride/partiallyhydrolyzed polyacrylamide mud.
 13. A method according to claim 1 furthercomprising circulating an activator system through said drill pipe andinto contact with said filter cake to set said filter cake andthereafter carrying out additional drilling.
 14. A method for drillingand cementing, comprising:drilling a wellbore with a drill stringcomprising a drill pipe utilizing a drilling fluid comprising water,drill solids and coal slag; circulating said drilling fluid down saiddrill pipe and up an annulus between said drill pipe and walls of saidborehole, thus laying down a filter cake on said walls of said boreholeduring said drilling and producing a used drilling fluid; withdrawingsaid drill string and inserting a pipe, thus creating an annulus betweensaid pipe and said walls of said borehole; incorporating an activatorsystem into said used drilling fluid to produce a cementitious slurry;and displacing said cementitious slurry into at least a portion of saidannulus between said pipe and said walls of said borehole.
 15. A methodaccording to claim 14 wherein said activator system comprises calciumoxide.
 16. A method according to claim 14 wherein said activator systemcomprises additional coal slag and calcium oxide or calcium hydroxide.17. A method according to claim 16 wherein said activator systemcomprises, in addition, at least one of lithium hydroxide, lithiumcarbonate, sodium silicate, sodium fluoride, sodium silicofluoride,magnesium hydroxide, magnesium oxide, magnesium silicofluoride, zinccarbonate, zinc silicofluoride, zinc oxide, sodium carbonate, sodiumbicarbonate, titanium carbonate, potassium carbonate, potassiumbicarbonate, sodium hydroxide, potassium hydroxide, potassium sulfate,potassium nitrate, potassium nitrite, sodium or potassium aluminate,calcium hydroxide, sodium sulfate, copper sulfate, calcium oxide,calcium sulfate, calcium nitrate and calcium nitrite.
 18. A methodaccording to claim 14 wherein said cementitious slurry is produced byalso incorporating blast furnace slag or fly ash into said used drillingfluid.
 19. A method according to claim 18 wherein said blast furnaceslag or fly ash is incorporated in an amount sufficient to give a weightratio of blast furnace slag or fly ash to coal slag within the range of1:99 to 50:50.
 20. A method according to claim 14 wherein said coal slagis water-quenched slag from the burning of lignite.
 21. A methodaccording to claim 14 wherein said drilling fluid comprises, inaddition, a polyalcohol component and a silicate.
 22. A method accordingto claim 21 wherein said polyalcohol component is a mixture of a solublepolyalcohol and an insoluble polyalcohol.
 23. A method according toclaim 22 wherein said soluble polyalcohol is a polyethercyclicpolyol.24. A method according to claim 14 wherein said drilling fluidcomprises, in addition, a viscosifier selected from the group consistingof biopolymers, silicates and starch.
 25. A method according to claim 14wherein, after said circulating of said drilling fluid, said drillingfluid is displaced and an activator system circulated through saidannulus between said drill pipe and said borehole wall to set saidfilter cake;said activator system is displaced with drilling fluid; andadditional drilling is carried out prior to said withdrawing of saiddrill string.
 26. A drilling fluid composition comprising water, drillsolids and 1 to 100 lbs/bbl of said composition of coal slag.
 27. Acomposition according to claim 26 comprising in addition a polyalcoholcomponent and a silicate component and wherein said coal slag is presentin an amount within the range of 20 to 50 lbs/bbl.
 28. A compositionaccording to claim 27 wherein said polyalcohol component is apolycyclicpolyetherpolyol of the formula: ##STR5## where x≧1 and y≧0.29. A composition according to claim 27 wherein said polyalcoholcomponent is a mixture of a soluble polyol and an insoluble alcohol. 30.A composition according to claim 27 wherein said polyalcohol componentcomprises an ethoxylated propoxylated polyol.
 31. A compositionaccording to claim 27 wherein said polyalcohol component comprises anepoxy-containing cyclic polyetherpolyol.
 32. A composition according toclaim 27 wherein said drilling fluid comprises, in addition, a secondaryfluid loss additive, a secondary weight material, a secondary shalestabilizer, and a copolymer viscosifer.
 33. A composition according toclaim 32 wherein said secondary fluid loss additive is selected from thegroup consisting of synthetic polymers, starch and bentonite clay, saidsecondary weight material is barite, said secondary shale stabilizer islime and said silicate is sodium silicate.
 34. A composition accordingto claim 26 wherein said drilling fluid comprises, in addition, aviscosifier selected from the group consisting of biopolymers, silicatesand starch.
 35. A composition according to claim 26 wherein said coalslag is water-quenched slag from the burning of lignite.